1. Field of the Invention
The present invention relates to an image reader and more particularly to an image reader for reading out an image signal by an image sensor which stores one scan line of image signal in a predetermined period.
2. Description of the Prior Art
An image reader by an image sensor such as a CCD linear image sensor which stores one scan line of image signal in a predetermined period has been widely used as an image reader for a facsimile or the like. A prior art method for reading the image in such an image reader is explained with reference to FIGS. 1(a) to 1(f), which show timings of signals in a read operation of the image reader by the prior art image readout method.
In a read mode, a start pulse shown in FIG. 1(a) is supplied to the linear image sensor from a control circuit of the image reader at a constant interval, and a charge corresponding to a light quantity sensed, that is, one scan line of image signal of an original document is stored in the linear image sensor, in synchronism with the start pulse, during a storage period between the start pulse and the next start pulse.
As shown in FIG. 1(b), the image signal stored in the linear image sensor in the immediately preceding storage period is discharged as a discharge or driving signal in a predetermined period shorter than the storage period and in synchronism with the start pulse, and it is supplied to a gate connected to an output terminal of the image reader.
When a read request pulse shown in FIG. 1(c) is applied, a gate signal shown in FIG. 1(d) is supplied from the control circuit to the gate in synchronism with the start pulse which immediately follows to the read request pulse. Thus, the gate is opened and the discharge signal of FIG. 1(b) is produced as an output signal shown in FIG. 1(e).
An original sheet drive signal pulse shown in FIG. 1(f) is supplied from the control circuit to an original sheet driver in synchronism with the start pulse immediately following the read request pulse. As a result, the original sheet is moved by one scan line width.
In the prior art method, as described above, the storage and the discharge of one line of image signal of the original sheet are effected in synchronism with the start pulses generated at constant intervals, and the image signal discharged in synchronism with the start pulse immediately following the read request pulse is read and the original sheet is moved by one scan line width in synchronism with the start pulse.
In this readout method, however, whether the image signal stored when the original sheet was stationary or the image signal stored while the original sheet was moving is produced, depends on the timing of the read request pulse.
Thus, in the prior art image readout method, the output image signals include both those stored while the original sheet was stationary and those stored while the original sheet was moving. Accordingly, the scan line density varies and proper image readout is no attained.